Consumers have, from their viewpoints, a number of demands concerning a good absorbent sanitary product, e.g. a napkin. The most important demands are that the napkin should be able to take up liquid and retain it during considerable time, also when subjected to compressive load. It is important that the wetting can be repeated, at which the napkin or the like on each wetting occasion can receive and distribute the liquid Within the napkin without leakage. Further, it should have an excellent absorption capacity. Furthermore, it should be soft and comfortable to wear, and must not contain any harmful or irritating agents. Moreover, the typical consumer appreciates that it has a high brightness. Yet, it must not be too expensive.
The demand specification above gives room for personal evaluations. Within the production branch there are yet a number of objective criteria for the quality determination of a fluff pulp. Thus the most important technical properties of a good fluff are:
--high absorption capacity and low time of absorption
--high specific volume wet
--high bulk after wetting, compression and spring back
--high rate of absorption
--high strength
--high brightness
Moreover the production requires that the pulp not contain or give off toxic, irritating or allergy-forming material, like e.g. formaldehyde, which is important not only for the consumer but also in the production.
In order to meet these requirements, various processes and ways to produce stiffer fibres to be used in fluff have been developed. The various ways are based on additives of various chemicals which work as a cross linking agent of the cellulose, i.e. to achieve chemical bonding between the cellulose molecules within the particular cellulose fibres, or the use of synthetic resins. Through the use of fibres, which are stiffer in wet condition, there is achieved a body of fluff giving increased absorption capacity and which moreover distributes the liquid throughout the whole volume of the fluff body. These properties of the fluff become of greater significance with the increased use of super absorbent polymers in products of type napkins and sanitary towels, which have resulted in a decreased total weight of the absorbent layer in these products.
U.S. Pat. No. 3,224,926 describes the use of formaldehyde for cross linking of cellulose in dry and defibrated state. However, formaldehyde as a cross linking agent is today questioned due to health aspects. The procedures described in the said patent moreover only have been examined in the laboratory and are not suited for implementation on a larger scale. Any commercial applications have not been realized based on this technology, as far as the applicant is aware.
EP-A-0 251 674, 0 251 676 and 0 252 650 describe the use of dialdehydes for dry chemical cross linking of defibrated fibres in the absence of fibre to fibre bondings. Dialdehydes, however, act in a similar way as formaldehyde, irritating to skin and mucous membranes. Even these reagents are therefor less suited in cellulose products which are to be used in close vicinity of the skin. As far as the applicant is aware of, nor have these processes been applied in a commercial scale.
EP-0 427 316 and 0 427 317 have recently proposed the use of polyvalent carboxylic acids, e.g. citric acid, for the cross linking of cellulose in fluff pulp at elevated temperatures. Some polyvalent carboxylic acids, however, have the disadvantage that they, at least during the preparation of the cellulose material, i.e. in the production line, are strongly irritating to the eyes. Some are moreover, at least at the present stage, very expensive and can therefor only be used for laboratory trials. These latter chemicals therefor have not achieved any commercial use as a cross linking agent for fluff pulp. Citric acid has been suggested as a cross linker and is commercially available, but the agent has an unfavourable influence on the final brightness of the cross linked fluff pulp.
Within the textile industry, the use of various cross linking chemicals to produce crease-proof cotton fabrics is well known, among them certain non-formaldehyde based reagents. Thus in June 1982, was proposed in Textile Research Journal, p. 357 ff, for said textile uses, additives of Glyoxal, polyacetal, bio-.beta.-hydroxy ethyl sulfon, propylene glycol diglycidyl ether and glyoxal adducts of urea and its derivatives, among them dihydroxy ethylene urea (DHEU), dihydroxy dimethylene urea (DHHEEU) and dihydroxy methylethylene urea (DHDMEU). U.S. Pat. No. 4,770,668 has later proposed more complex derivatives of dihydroxy ethylene urea, still for the use as a crease-proof agent, i.e. as permanent press agents in cellulose based fabrics and fabrics made of cellulose/polymer mixtures.
In spite of large efforts in the US as well as in Europe and Japan within the area of development of absorbent sanitary products, no technically and commercially completely acceptable fluff pulp with chemical cross linkings between the cellulose molecules has been presented so far.